Microcapsules can be constructed of various types of wall or shell materials to house varying core material for many purposes. The encapsulation process is commonly referred to as microencapsulation. Microencapsulation is the process of surrounding or enveloping one substance, often referred to as the core material, within another substance, often referred to as the wall, shell, or capsule, on a very small scale. The scale for microcapsules may be particles with diameters in the range between 1 μm and 1000 μm that consist of a core material and a covering shell. The microcapsules may be spherically shaped, with a continuous wall surrounding the core, while others may be asymmetrical and variably shaped.
General encapsulation processes include emulsion polymerization, bulk polymerization, solution polymerization, and/or suspension polymerization and typically include a catalyst. Emulsion polymerization occurs in a water/oil or oil/water mixed phase. Bulk polymerization is carried out in the absence of solvent. Solution polymerization is carried out in a solvent in which both the monomer and subsequent polymer are soluble. Suspension polymerization is carried out in the presence of a solvent (usually water) in which the monomer is insoluble and in which it is suspended by agitation. To prevent the droplets of monomers from coalescing and to prevent the polymer from coagulating, protective colloids are typically added.
Through a selection of the core and shell material, it is possible to obtain microcapsules with a variety of functions. This is why microcapsules can be defined as containers, which can release, protect and/or mask various kinds of active core materials. Microencapsulation is mainly used for the separation of the core material from the environment, but it can also be used for controlled release of core material in the environment. Microcapsule walls can also act as a barrier, separating a two-component system, where one constituent is in the core and the other is in the environment surrounding the capsule, such as being within a matrix in which the capsules are located. A disadvantage to a capsule/matrix system is that they are typically not cost effective because the matrix requires more material of the second reagent or catalyst to be present than is actually necessary for a typical reaction to occur. Furthermore, an even dispersion of the capsules in the matrix (or medium) is required, and is not easily achieved when considering the differences in density of the materials and/or the size of the capsule. Furthermore, often times a specific stoichiometry of reagents is required, and having one reagent as the matrix would limit the number of reactions that can be carried out.
Another way to separate two components is by synthesizing two different capsules, one with a first reagent in it and the other with a second reagent in it. When the capsules are ruptured by damage, the intended effect is triggered through the release and reaction of the reagents. After release, the reagent is depleted, leading to a singular local event. The main issue with having two separate capsules is that both capsules would need to be ruptured simultaneously within a reasonable distance from one another in order for a reaction to occur. Furthermore, an even dispersion of the two capsules is required, which is not easily achieved when considering the differences in the density of materials and/or the size of the two capsules.
Since the development of microcapsules, there has been a constant need for improved microcapsules. Further, self-healing formulations have been researched for at least the past decade in an attempt to restore the physical and mechanical integrity of a surface quickly after damage occurs without human intervention. In particular, there is a need to develop a two-component microcapsule system that separates two reagents in a more compact unit, in close proximity until the time is needed for their interaction, especially ones that can provide a self-healing formulation.